Gas springs are utilized in many different types of equipment for absorbing and cushioning forces. These gas springs have been extensively utilized in the automobile industry in association with presses to cushion the large pressing forces. Such springs, one example of which is illustrated in U.S. Pat. No. 3,101,194 (W. E. Hennells, Sr.), have typically utilized rather low pressure to permit the spring to perform a large number of working cycles. This low internal pressure has required the use of larger cross-sectional areas, and the use of a substantial number of air springs disposed in parallel relationship, in order to effectively cushion large pressing forces. Use of such gas springs is hence often more complex than desired, and in some instances such usage is impossible due to space or other operational limitations.
Because of the desired performance which can be achieved by gas springs, specifically on presses, numerous attempts have been made to increase the internal pressure of the gas spring so as to substantially increase the force-cushioning capacity and/or reduce the overall spring size. Thus, numerous manufacturers have attempted to provide a gas spring having a normal internal gas pressure in the range of 2,000 to 3,000 psi. However, most of these springs have proven wholly unsuccessful since the springs have been observed to rapidly permit leakage of the pressurized gas past the stuffing box associated with the piston rod, and hence the spring fails after a relatively small number of operational cycles.
In an attempt to provide a high-pressure gas spring which would permit successful operation over a larger number of cycles, such as at least 1,000,000 cycles, Applicant has extensively studied the performance of gas springs, and specifically the typical failure point, namely the packing between the housing and the piston rod. It has been observed that, when the spring pressure exceeds about 1,500 psi, the rubber sealing ring as typically associated with the packing undergoes rapid failure since sealing rings constructed of rubber and similar products are unable to successfully perform a large number of cyclic operations under pressure levels in excess of about 1,500 psi. Such rubber sealing rings have been observed to rapidly wear and deteriorate when operated under high pressures, apparently due to the gas permeating the porosity of the rubber and becoming temporarily trapped therein.
It has also been observed that the typical stuffing box bushings as provided between the housing and piston rod perform unsatisfactorily. It is common to utilize bronze bushings for slidably supporting the piston rod. However, this bushing is unable to provide sufficient lubrication between the elastomeric sealing ring and the piston rod, and hence the sealing ring wears rapidly due to lack of lubrication.
Experimental evaluation of high-pressure gas springs employing packings formed by bronze bushings and conventional rubber or elastomeric sealing rings has indicated the inability of such gas springs to successfully operate for a large number of cycles due primarily to excessive wear of the sealing ring.
In apparent recognition of the failure of the packing structure, there has been developed one high-pressure gas spring which appears to have the capability of operating for a large number of cycles. This spring, known as the "Kaller" spring and manufactured by Stroemsholmens Mekaniska Verkstad AB, uses an internal gas (nitrogen) maintained at a relatively high pressure while allegedly permitting a large number of operational cycles. In this Kaller spring, there is provided a special overpressure seal associated with the packing, which seal has a higher pressure than the nitrogen pressure so as to prevent gas leakage from the unit. This special packing involves a pair of opposed portions defining thereon lips for slidably engaging the piston rod, which packing is axially compressed by the nitrogen within the unit through a piston arrangement which magnifies the pressure so that the packing ring has a higher pressure imposed thereon in an attempt to create a long-wearing sliding seal with the piston rod. The overall arrangement of this Kaller spring, however, and specifically the packing structure therefor, is complex and is believed less dependable than desired.
This invention relates to a gas spring which incorporates an improved packing structure capable of providing a lubricated guide and seal for slidable engagement with the piston rod so as to permit the gas spring to contain therein a high-pressure gas while at the same time enabling the spring unit to successfully function for a large number of operational cycles. The packing structure includes a pair of lubricating-type bushings disposed for slidable engagement with the piston rod, which bushings are axially spaced and confine a seal ring therebetween, the latter preferably being of polyurethane. The bushings are preferably impregnated with a lubricant such as graphite to effect lubrication of not only the bushings, but also of the slidable contact area between the piston rod and seal ring to minimize wear and hence enable the unit to operate for a large number of cycles. The sealing ring, by being constructed of a stiff material such as urethane, can withstand exposure to the high-pressure gas within the unit and still successfully operate for a large number of cycles. The innermost bushing preferably abuts against a shoulder on the housing, and not against the seal ring, to avoid imposition thereon of excessive compressive forces, whereby the seal ring remains in its desired configuration so as to effectively sealingly engage the piston rod.
It is thus an object of this invention to provide an improved gas spring which employs a packing ring capable of operating with a high-pressure gas, such as in the order of 2,000 to 3,000 psi, and capable of operating for a large number of cycles, such as 1,000,000 cycles.
A further object is to provide an improved gas spring, as aforesaid, which utilizes a packing structure which is of substantial simplicity, such as by employing a pair of bushings which are axially spaced and have a seal ring therebetween, whereby the packing structure is relatively economical to manufacture and assemble.
A further object is to provide an improved gas spring, as aforesaid, which is capable of absorbing or cushioning large external forces while at the same time enabling the spring unit to be of minimal size and complexity, which spring unit can be either self-contained or connected through conduits to a surge tank containing pressurized gas so as to positively maintain a constant base pressure within the spring unit.
Other objects and purposes of the invention will be apparent to persons familiar with structures of this general type upon reading the following specification and inspecting the accompanying drawings.